SugarCube's Insomniac Asylum

Years ago I read a book about dragons that maintained that their metabolism produced methane that filled out their bodies like balloons. This explained how they could fly (only small wings being necessary to guide the lighter-than-air contraptions they were and also the fiery breath (methane being highly flammable). The dragons were set in a world so fascinating that I wouldn't mind visiting it. It was full of things like cities built on islands above waterfalls and blasted areas where the bad dragons lived.

Normally I don't have much time for dragons but I do like serious theories that explain their various powers. All that plus an attractive world, fabulously illustrated, and the author couldn't really lose!